Flared laser oscillator waveguide

ABSTRACT

A broad area semiconductor diode laser device includes a multimode high reflector facet, a partial reflector facet spaced from said multimode high reflector facet, and a flared current injection region extending and widening between the multimode high reflector facet and the partial reflector facet, wherein the ratio of a partial reflector facet width to a high reflector facet width is n:1, where n&gt;1. The broad area semiconductor laser device is a flared laser oscillator waveguide delivering improved beam brightness and beam parameter product over conventional straight waveguide configurations.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/855,710 filed Sep. 16, 2015, which is a continuation of U.S. patentapplication Ser. No. 14/011,661 filed Aug. 27, 2013, now U.S. Pat. No.9,166,369, issued Oct. 20, 2015, which claims the benefit of U.S.Provisional Patent Application 61/810,261 filed Apr. 9, 2013, all ofwhich are incorporated herein by reference in their entirety for allpurposes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Generally, the field of the present invention is semiconductor diodelasers. More particularly, the invention relates to flared laseroscillator waveguides.

2. Background

Multimode laser diodes, also known as broad area lasers (BALs), have theproperty that their slow-axis beam-parameter-product (BPP) and theirslow-axis brightness (power÷BPP) degrade progressively when they aredriven at higher current to generate higher power. Brightness can beimproved in BALs by reducing the emitter width; however, the current atwhich the maximum brightness occurs also happens at progressively lowercurrent values. Hence, the maximum output power at the maximumbrightness also drops. For power-scaling applications and reducing thecost-per-watt of producing diode lasers, higher brightness at higheroutput power per emitter is very desirable.

Semiconductor diode lasers are formed by growing multiple layers ofsemiconductor materials on a suitable substrate with a lattice constantthat allows choice of materials to produce desired emission wavelengths.A typical semiconductor laser comprises n-type layers, p-type layers andan undoped active layer between them such that when the diode isforward-biased, electrons and holes recombine in the active region layerto produce light. The active layer (quantum well(s), quantum wire(s) orquantum dots, type-II quantum well(s)) resides in the waveguide layerwhich has a higher index of refraction compared to the surrounding p-and n-doped cladding layers. Light generated from the active layer isconfined in the plane of the waveguide.

A conventional edge-emitting Fabry Perot broad area laser diode isarranged as a rectangular gain or index-guided semiconductor structure.Opposing end facets of the waveguide define high and partial reflectorsto provide feedback for oscillation of light within the resonator. Themulti-layered semiconductor laser diode structure extends the length ofthe laser and has a broad width for electrical injection extending toopposite side surfaces which also extend the length of the laser. Themulti-layered semiconductor materials are typically arranged so that thelaser operates in a single mode along the growth direction of the laserand this direction is defined as fast-axis direction. Since along thefast-axis direction the semiconductor laser operates in a single mode,the brightness of laser diode in this direction cannot be improved anyfurther—it is so called diffraction-limited. The distance between thetop and bottom surfaces of the multi-layered semiconductor laserstructure thus provides the smaller dimension of the end facets, i.e.,the thickness of the stripe, typically on the order of microns. On theother hand, the width of the multi-layered laser structure provides thelarger dimension of the end facets, i.e., the stripe-width is typicallyon the order of many tens of microns to hundreds of microns. Because thestripe width is much larger than the wavelength of light, the lateralproperty of an optical field propagating along the optical axis of thewaveguide is highly multimode along the longer stripe dimension and thecorresponding axis is described as slow-axis.

Diode laser ridge waveguide structures with single-mode structuralcharacteristics across the slow-axis have been described which may besuitable for lower powers where single-mode performance is desirable.For example, in U.S. Pat. No. 6,014,396 to Osinki et al. a flaredsemiconductor optoelectronic device is disclosed that has adouble-flared structured. Other examples of conventional ridge waveguidestructures can be found in U.S. Pat. Nos. 7,623,555 and 6,798,815. Thesedevices have single mode beam quality in both directions but suchperformance comes at the expense of limited output power. However, theproblem of scaling to higher powers while maintaining superiorbrightness continues to pose a challenge in the art of diode lasers,particularly where devices are highly multimode across the slow axis,and so a need remains for improvements associated therewith.

SUMMARY OF THE INVENTION

Accordingly, the present invention satisfies the aforementioned need byproviding an innovation in broad area semiconductor diode lasertechnology which includes providing a flared laser oscillator waveguide(FLOW) with a flared current injection region extending and wideningbetween a multimode high reflector facet and a partial reflector facet.By narrowing the width of the electrically-pumped stripe towards thehigh reflector facet, the higher order modes with higher divergenceangles are prevented from coupling back into the laser. As a result, theslow-axis divergence of the laser is smaller compared to a device withrectangular geometry having the same width for the partial reflector.

Furthermore, light propagating in the flared current injection regioncan form a thermal waveguide that is closer to the width of thenarrower, high reflector side causing a beam output at the partialreflector facet to have a substantially narrower beam width than thepartial reflector facet width. As a result, the-beam-parameter-product,BPP (slow-axis near-field width times the slow-axis divergence) issmaller for FLOW devices compared to BAL devices. Since the near-fieldis smaller than the physical width at the partial reflector side, FLOWdevices can be designed to have a larger total area compared to BALwithout sacrificing BPP. The enlarged total pumped area provided by theflaring of the flared current injection region serves to reduce thermalresistance and electrical series resistance in the device, resulting inhigher electrical-to-optical power conversion efficiency. This leads tohigher output power at a given operating current compared to BALdevices. Higher power and smaller BPP leads to increased beam brightnessin the slow-axis.

In addition to the application to broad area diode lasers, the FLOWconcept can also be applied to other types of semiconductor-basedFabry-Perot lasers, such as quantum cascade laser (QCL), interbandquantum cascade lasers (IQL), by way of example. Broad area diode laserswith flared laser oscillator waveguides can also find particular use inlaser diode modules, which can be configured for various applicationssuch as fiber-coupling or direct pumping.

Thus, in one aspect of the present invention, a broad area semiconductordiode laser device includes a multimode high reflector facet, a partialreflector facet spaced from the multimode high reflector facet, and aflared current injection region extending and widening between themultimode high reflector facet and the partial reflector facet, whereinthe ratio of a partial reflector facet width to a high reflector facetwidth is n:1, where n>1.

In another aspect of the present invention, a multimode flared laseroscillator waveguide includes a semiconductor gain volume having amultimode high reflector and an output coupler oppositely disposed andspaced apart by a resonator length, top and bottom oppositely disposedsides spaced apart by a resonator height, and first and secondoppositely disposed flared sides spaced apart by a variable resonatorwidth providing the high reflector with a shorter width than the outputcoupler.

In another aspect of the present invention a flared laser oscillatorwaveguide includes a semiconductor gain volume which includes a highreflector surface and an opposing partial reflector surface spaced apartfrom each other by a resonator length, top and bottom opposite surfacesspaced apart by a resonator height, and first and second opposite sidesurfaces spaced apart by a resonator width, wherein at least a portionof the opposite side surfaces are spaced apart by a variable resonatorwidth forming a flared oscillator region and providing the highreflector surface with a shorter width than the partial reflectorsurface.

The foregoing and other objects, features, and advantages will becomemore apparent from the following detailed description, which proceedswith reference to the accompanying figures, which are not necessarilydrawn to scale.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of flared laser oscillator waveguide devicein accordance with an aspect of the present invention.

FIG. 2 is a perspective view of an optical resonator of a flared laseroscillator waveguide device in accordance with an aspect of the presentinvention.

FIG. 3 is a chart of slow-axis (SA) beam parameter product (BPP) forconventional broad area diode laser devices and flared laser oscillatorwaveguide diode laser devices in accordance with aspects of the presentinvention.

FIG. 4 is a chart of slow-axis (SA) brightness for conventional broadarea diode laser devices and flared laser oscillator waveguide diodelaser devices in accordance with aspects of the present invention.

FIG. 5 is a chart showing near field beam width shrinking as a functionof operating power for beams emitted from flared laser oscillatorwaveguide diode laser devices in accordance with aspects of the presentinvention compared to a broad area laser.

FIG. 6 is a chart showing far field beam divergence as a function ofoperating power for beams emitted from conventional broad are diodelaser devices and flared laser oscillator waveguide diode laser devicesin accordance with aspects of the present invention.

FIG. 7 is a chart showing optical power (Power) as well aselectrical-to-optical power conversion efficiency (Efficiency) versuscurrent curves for flared laser oscillator waveguide devices of thepresent invention and conventional broad area laser diodes.

FIGS. 8A-C show top cross-sectional views for three alternative currentinjection regions in accordance with aspects of the present invention.

FIGS. 9A-C show top cross-sectional views for three alternative currentinjection regions in accordance with aspects of the present invention.

FIG. 10 is a three dimensional chart showing current and brightness fordifferent facet width ratios in accordance with an aspect of the presentinvention.

FIGS. 11A-C show top cross-sectional views for three alternative currentinjection regions and additional higher order mode discriminatingfeatures in accordance with aspects of the present invention.

FIGS. 12A-B show top cross-sectional views for two alternative currentinjection regions and wavelength-stabilizing grating in accordance withaspects of the present invention. 12A shows distributed feedback (DFB)configuration and 12B shows distributed Bragg reflection (DBR)configuration.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, a perspective view is shown of a first embodimentof a broad area flared laser oscillator waveguide (FLOW) device,generally designated 10, in accordance with an aspect of the presentinvention. The device 10 includes a current injection region 12 forelectrical pumping, the region 12 having a trapezoidal shape extendingbetween a high reflecting back facet 14 and a partial reflecting frontfacet 16. The device 10 can have a ridge, or mesa, shaped structure 18as depicted in FIG. 1 forming an index-guided region or the shape 18depicted in FIG. 1 can be gain-guided. The device 10 is configured foremission of a laser beam 20 out of the front facet 16 thereof. A beamspot 21 is formed on the front facet 16 of the device 10 as the beam 20is emitted therefrom. Ridge structures, particularly active portionsthereof, can be made in part from a variety of different conventionalsemiconductor materials typically grown in layers through conventionalsemiconductor deposition processes. Exemplary materials include GaAs,AlGaAs, InGaAsP, InGaAs, InP, other elements in the III & V columns, andvarious combinations thereof. Suitable deposition processes can includeCVD, MOCVD, and MBE.

With additional reference to FIG. 2, shown within the ridge structure 18is an active region 22 formed by the layered semiconductor material. Theactive region 22 is disposed in, forms a portion of, or defines anoptical resonator 24 in which light may oscillate along an optical axis26 to become amplified. The resonator 24 includes aforementioned backand front facets 14, 16, as well as opposite sides 28, 30. In someexamples, the resonator 24 also includes opposite upper and lowersurfaces 32, 34 which are coextensive with the current injection region12 in the device 10. The length of the resonator 24 can be selected fordifferent purposes, such as the end-use application, manufacturingrequirements, or optimization requirements. Suitable lengths can include1 mm or less, 3 mm, 5 mm, 10 mm or more, or other variations thereof.The high reflecting back facet 14 has a narrower width ‘a’ than a width‘A’ of partial reflecting front facet 16. Importantly, in examplesherein both facets 14, 16 have widths that are highly multimode. Thus,for optical wavelengths around 1 μm (e.g., 976 nm), the back facet 14can have a minimum width ‘a’ as low as approximately 10 μm, but ispreferably around 30 to 75 μm, with other examples also being discussedherein. Other wavelengths are also possible, resulting in differentwidths, lengths, or other dimensions. Suitable reflectivities for highreflecting back facet 14 includes a reflectivity of 99% or more, but thereflectivity can be selected to be lower as needed. The partialreflecting front facet 16 couples light out of the optical resonator 24and has a larger width typically associated with conventional broadstripe diode lasers. For example, suitable widths ‘A’ for the frontfacet 16 include 25 μm, 50 μm, 75, μm, 150 μm, or larger. The thicknessof the facets 14, 16, as well as the thickness of the remainder of theoptical resonator 24 is typically uniform, and on the same order as theoptical wavelengths. For optical wavelengths of around 1 μm, thethickness of the stripe is typically on the order of a couple ofmicrons. For example, one such device can include a 0.75 μm n-cladding,a 1.5 μm waveguide with quantum well imbedded therein, 1 μm p-waveguide,and 0.1 μm highly doped contact layer. Variations in thickness are alsopossible. Typical reflectivities for the partial reflecting front facet16 include between 0.05% and 15%, but may be selected or tuned as neededin accordance with the desired output characteristics of the device 10.

Representative beam 20 is also shown being emitted from front facet 16of optical resonator 24 in FIG. 2. The beam 20 is highly divergentacross a fast axis 36 and has a relatively slow divergence across a slowaxis 38. The beam 20 is highly divergent across fast axis 36 due to thesmall thickness of the resonator 24. The beam 20 is slowly divergentacross slow axis 38 due to the relatively large minimum width ‘a’ of theresonator 24. Collimation and redirection optics (not shown) can bepositioned in the path of the emitted beam 20 to collimate and directthe beam 20 for subsequent application, such as combining beam 20 withother diode laser beams for coupling into an optical fiber or gainmedium.

The beam parameter product (BPP) and beam brightness are importantcharacteristics for laser pumping and for other applications of thedevice 10. The beam parameter product is a measure of beam quality andis given by the product of the divergence angle of a beam with its waistradius. Minimum beam parameter products are desirable for manyapplications. In typical broad stripe diode structures slow axis BPPincreases as injected current increases due to increase in far-fielddivergence angle, leading to less desirable beam characteristics as thediodes are driven to higher output powers. Beam brightness is a measureof diode performance and is given by the quotient of beam power and BPP.A higher brightness is desirable for many laser applications,particularly for higher power applications like brightness conversion infiber lasers. It is also important for optically coupling light intofibers more generally. Brightness is typically approximately flat orincreases somewhat as a function of input current for conventional broadarea laser diodes.

For example, a BPP-current relation 40 is shown in FIG. 3 for the slowaxis of beams emitted from four conventional broad area laser diodeshaving a constant width (i.e., ‘a’=‘A’) of 150 μm along the lengthsthereof. The relation 40 shows a BPP of approximately 6 mm-mrad at 8amps which rises steadily to 10 mm-mrad at 20 amps. In contrast, aBPP-current relation 42 is shown for three example devices 10 having an‘a’ dimension of 30 μm for high reflecting back facet 14 and an ‘A’dimension of 150 μm for partial reflecting front output facet 16 and aconstant linear change in resonator width therebetween. The BPP of beamsfor the three example flared devices 10 is approximately 4 mm-mrad at 8amps up to approximately 16 amps where BPP rises steadily toapproximately 6 mm-mrad at 20 amps. Thus, devices 10 in accordance withaspects of the present invention are operable to deliver enhanced BPPperformance compared with conventional broad area laser diodes over aportion or the entirety of the diode laser device operational range. Insome examples, and also in relation to input current, devices 10 canprovide 10%, 20%, or even 50% or more of improvement in BPP overconventional broad area laser diodes.

In addition to substantial improvement in BPP, brightness of devices 10in accordance with aspects of the present invention can also experiencesubstantial gains in unexpected fashion. For example, abrightness-current relation 44 is shown in FIG. 4 for the slow axis ofbeams emitted from the four conventional broad area laser diodesdescribed with reference to FIG. 3 above. The relation 44 showsbrightness in the range of approximately 1.2 to 1.8 W/mm-mrad from 8amps to 20 amps. In contrast, a brightness-current relation 46 is shownfor the three example devices 10 described with reference to FIG. 3above. The brightness of beams for the three example flared devices 10is approximately 2 W/mm-mrad at 8 amps increasing to over 3 W/mm-mrad at14 amps and descending to approximately 2.4 W/mm-mrad at 20 amps. Thus,devices 10 in accordance with aspects of the present invention areoperable to deliver enhanced brightness performance compared withconventional broad area laser diodes over a portion or the entirety ofthe diode laser device operational range and for similar aperture sizes.In some examples, and also in relation to input current, devices 10 canprovide 10%, 20%, 50%, or even 100% or more of improvement in brightnessover conventional broad area laser diodes.

The substantial improvements in BPP and brightness can be attributed inpart to the near field performance of beams emitted by devices 10. FIG.5 is a plot of full-width at 1/e² value of the normalized intensityprofiles across the slow axis of beams emitted by a device 10 withdimensions described with reference to FIGS. 3 and 4 for differentselected power levels ranging from 2 watts to 14 watts. It can be seenthat the widths of the beams are consistently smaller by 20% or more anddecrease more rapidly for flared laser oscillator waveguide diodes 48compared to 150 μm broad area lasers 49 as the power increases.Moreover, with additional reference to FIG. 6, the slow-axis far-fielddivergence 50 of flared laser oscillator waveguide devices start atabout 8 degrees at full-width at 1/e² value and remain nearly constantfrom threshold to 14 watts. For this same operating power range, a 75 μmBAL device slow-axis far-field divergence 51 increases non-linearly from8 degrees at full-width at 1/e² value at 2 watts to over 18 degrees atfull-width at 1/e² value at 14 watts. Improved BPP over conventionaldevices is attributed to a smaller amount of far field bloom seen in theemitted beam 20 as well as narrower near-field profile compared to BALs.The reduction in near-field bloom can be associated with the increasedoptical intensity of the beam 20 at the flared front facet 16 and itseffective width has narrowed due to guiding and mode stripping caused bythe tapered back facet 14. Thus, the output beam 20 typically emits in aspot 21 from the front facet 16 across less than the whole width ‘A’thereof.

By selecting the HR back facet 14 to have a narrower width than the PRfront facet 16 (i.e., a<A), lateral mode control is introduced into thedevice 10. Also, the HR back facet 14, as opposed to the PR front facet16, is selected to have a narrower width since higher order modesreflected at the facet 14 are diffracted at an angle such that thehigher order modes do not propagate back into the electrically-pumpedregion of the device 10. Accordingly, fewer lateral optical modes arepropagated in a device 10 across the slow axis compared to aconventional straight broad area laser diode having the same width ‘A’for PR output facet 16. Additionally, as the fewer mode light propagatesback through the resonator 24, a thermal waveguide is formed thereinrunning the length of the resonator 24 and having a width that is closerto the width ‘a’ of the narrower high reflecting back facet 14. Thecorresponding narrower thermal waveguide limits the effective spot sizeof the beam 20 to a substantially narrower spot 21 as the beam exits thefront facet 16. The substantially narrower spot 21 can be narrower by5%, 20%, 50% or more, for example, and is typically dependent on theinput current to the device 10, as illustrated in FIG. 5. The fewer modethermally guided light emits as a beam 20 that has higher slow axisbrightness than conventional broad area laser diodes having the similarexit aperture width. As will be seen hereinafter, due to the lateralmode control introduced by the back facet 14 aperture, the totalcurrent-injected area of the device 10 can be optimized to lower boththe thermal and electrical resistance thereof for improved performance.Moreover, by flaring the shape of the optical resonator 24 and currentinjection region 12, the total electrically-pumped area is an enlargedarea that does not compromise slow-axis BPP thereby improving theoverall thermal resistance and electrical series resistance of thedevice 10. Consequently, devices 10 achieve higher peak efficiencycompared to conventional broad area diode lasers with equal outputaperture size yet produce higher output power at the same brightness asillustrated in FIG. 7. Since the size of the output beam 20 is notdetermined by the pumped output aperture width, the effective area ofdevices 10 can be larger and therefore the series resistance of thedevices 10 can be commensurately lower.

Referring to FIG. 7 a chart is shown of output optical power andelectrical-to-optical power conversion efficiency (PCE) as a function ofinput current for a device 10 having a 30 μm to 150 μm flared currentinjection configuration and a conventional BAL with a constant width of75 μm, both devices having a 5 mm cavity length. The output opticalpower 52 for the 75 μm BAL performs similar to or slightly worse thanthe output optical power 53 for a flared device 10. The PCE, designated54, for the 75 μm BAL depicts a similar to or slightly worse result thanthe PCE, designated 55, for the flared device 10.

Referring now to FIGS. 8A-8C, there are shown several examples ofcurrent injection regions of alternative embodiments of FLOW devices.With particular reference to FIG. 8A, a top view is shown of atrapezoidal perimeter of a current injection region 56 of an alternativeembodiment of a flared laser oscillator waveguide device in accordancewith an aspect of the present invention. The current injection region 56has a narrower width for a high reflecting back facet 58, a larger widthfor a partially reflecting front facet 60, and segmented flat opposingside surfaces 62, 64 extending between the facets 58, 60. The currentinjection region 56 includes a plurality of flared regions 66 ofdifferent widths, though each flared region 66 is wider than the highreflecting back facet 58. In FIG. 8B, a top view is shown of a perimeterof an inward curved current injection region 68 of another alternativeembodiment of a flared laser oscillator waveguide device in accordancewith an aspect of the present invention. The current injection region 68has a narrower width for a high reflecting back facet 70, a larger widthfor a partially reflecting front facet 72, and a pair of smooth flaredside surfaces 74, 76 extending between the facets 70, 72. In FIG. 8C, atop view is shown of a perimeter of an outward curved current injectionregion 78 of another alternative embodiment of a flared laser oscillatorwaveguide device in accordance with an aspect of the present invention.The current injection region 78 has a narrower width for a highreflecting back facet 80, a larger width for a partially reflectingfront facet 82, and a pair of smooth flared side surfaces 84, 86extending between the facets 80, 82. Various combinations of shapesdescribed for regions 56, 68, 78 are also possible.

Referring now to FIGS. 9A-9C, additional examples are shown of currentinjection regions which are similar to the regions shown in FIGS. 8A-8Cand to which reference shall be made with respect to like numerals.Thus, in FIG. 9A, a top view of a current injection region 88 is shownfor an alternative embodiment of a flared laser oscillator waveguidedevice, the region 88 being similar to current injection region 56having a plurality of flared regions 66 to the extent that region 88also includes a plurality of flared regions 66. Region 88 also includesnarrower and wider end rectangular portions 90, 92 extending from therespective opposite narrower and wider end regions 66 of region 56. Thenarrower end rectangular portion 90 extends a predetermined distanceallowing a high reflecting back facet 94 to be formed, e.g., throughcleaving along a cleave plane 96, that has a well-defined aperture.Because the rectangular portion 90 has a constant width parallel to thecleaving plane 94, variation in the location of the cleaving plane 94does not affect the selected width of the back facet 94. The wider endrectangular portion 92 extends a predetermined distance allowing apartially reflecting front facet 98 to be formed, e.g., through cleavingalong a cleave plane 100, that also has a well-defined aperture. In FIG.9B, a current injection region 102 has a inwardly curved middle portionextending between a narrower end rectangular extension 104 and a widerend rectangular extension 106. The rectangular extensions 104, 106extend predetermined distances allowing respective high reflecting backand front facets 108, 110 to be formed at respective cleaving planes112, 114, so as to provide the formed facets 108, 110 with well-definedapertures. In FIG. 9C, a current injection region 116 has a outwardlycurved middle portion extending between a narrower end rectangularextension 118 and a wider end rectangular extension 120. The rectangularextensions 118, 120 extend predetermined distances allowing respectivehigh reflecting back and front facets 122, 124 to be formed atrespective cleaving planes 126, 128, so as to provide the cleaved facets122, 124 with well-defined apertures.

With respect to embodiments described in FIGS. 9A-9C, the varioussegmented and curved shapes can be combined in various ways, andrectangular extensions can be added or defined for one or both ends of acurrent injection region of a device. The rectangular extending portionscan be advantageous in manufacturing by providing predictability withrespect to the apertures of the back and front facets. A cleaving planecan be coplanar or approximately coplanar with the defined exposed endof the corresponding rectangular extending portion, or alternatively thecleaving plane can be as depicted in FIGS. 9A-9C at a distance from thedefined exposed end along the predetermined length of the rectangularextending portion. Thus, while error may be allowed in the preciselocation of a cleave plane, the well-defined width of the facet ismaintained. Moreover, the cleave planes and corresponding facets formedthereby need not be perpendicular to the current injection region oroptical axis thereof allowing for angular cleaves, etc.

Various examples of the flared laser oscillator waveguide devices inaccordance with the present invention can be gain-guided or index-guidedwhich can be implemented in different ways, though the methods describedherein are not intended as exhaustive. For example, in a gain-guideddesign, a p-contact can be delineated in accordance with the top viewcurrent injection region perimeters described in FIGS. 8A-9C. Thepattern of the p-contact is formed by making an opening in one or morelayers of dielectric of the flared laser oscillator waveguide device 10.A p-contact is then deposited to form the pattern as describedhereinabove. Alternatively, a deposited p++ doped contact layer can beetched away where the contact is not desired, i.e., outside of a currentinjection region perimeter, so as to define a current-blocking Schottkybarrier. One suitable way to fabricate an index-guided design includesetching away deposited semiconductor material down a predetermineddistance, such as 0.5 μm, 1 μm, 2 μm, or another selected thicknessdependent on the structure of the device 10. By etching away thesemiconductor material outside the current-injected area, an indexcontrast is introduced at the etched step in the lateral (slow-axis)direction

In FIG. 10 is shown a three dimensional optimization curve 130 depictingmultiple flared laser oscillator waveguide devices 10 having a constantchanging current injection region width, such as depicted in FIGS. 1-2,but for different ratios of back facet width to front facet width.Current and slow-axis brightness are also axes for the curve 130 so thatcorresponding optimized designs can be understood for specified rangesof brightness or injection current. Accordingly, in some examples, thewidths of the back and front facets are selected in accordance with anoptimized facet width ratio.

FIGS. 11A-11C illustrate top cross-sectional views of additionalembodiments of flared laser oscillator waveguide devices in accordancewith aspects of the present invention. In FIG. 11A a flared currentinjection region 132 is shown extending between a high reflecting backfacet 134 with a width ‘a’ and partial reflecting front facet 136 with awidth ‘A’. A pair of scattering elements 138 is oppositely positioned inthe current injection region 132 and extends between the back facet 134and the front facet 136. The scattering elements 138 each have aselected width with respect to the width ‘A’ of the front facet 136 suchthat a portion 140 of the front facet 136 which does not have scatteringelements 138 associated therewith has a smaller width ‘g’.

Difference between back facet width ‘a’ and portion width ‘g’ is alsopossible, as illustrated in the alternative embodiments shown in FIGS.11B and 11C. In FIG. 11B, a flared current injection region 142 also hasa back facet 144 with corresponding width ‘a’ and front facet 146 with acorresponding width ‘A’. Current injection region 142 includes lateralscattering elements 148 extending between the back facet 144 and frontfacet and defining a portion 150 of the front facet 146 with a width ‘g’where scattering elements 148 are not present at the interface thereof.Also, scattering elements 148 include a non-linear variation in width,here an interior curved contour, extending between the back and frontfacets 144, 146.

In FIG. 11C, a flared current injection region 152 also has a back facet154 with corresponding width ‘a’ and front facet 156 with acorresponding width ‘A’. A pair of scattering elements 158 is oppositelypositioned in the current injection region 152 and extends from thefront facet 146 to a predetermined distance along the length of thecurrent injection region 152. Also, the scattering elements 158 eachhave a selected width with respect to the width ‘A’ of the front facet156 such that a portion 160 of the front facet 156 which does not havescattering elements 158 associated therewith has a smaller width ‘g’. Asit will be appreciated by those with skill in the art in view of thisdisclosure, different variations and combinations of the scatteringelements described in FIGS. 11A-11C are possible, includingincorporation of other aspects of the present invention hereindescribed.

Various scattering patterns, such as scattering elements 138, 148, 158,are defined in flared laser oscillator waveguide devices of the presentinvention in order to introduce loss of higher order modes of laserlight propagating therein for improved beam output. While differentgeometric examples are described, the scattering patterns can generallybe configured to overlap the modal content of the laser light to achievehigher order mode suppression. Scattering patterns can be formed in avariety of ways to realize mode-stripping effects, including thenon-resonant grating, formation of micro-structures that includefeatures with index contrast, or formation of a second-order grating, inthe selected patterned area.

Referring now to FIGS. 12A-12B there are shown additional embodiments offlared laser oscillator waveguide devices in accordance with aspects ofthe present invention. In FIG. 12A a top cross-sectional view of acurrent injection region 200 of a flared laser oscillator waveguidedevice is shown which is configured to be wavelength stabilized. Currentinjection region 200 includes narrower high reflecting back facet 202having a width ‘a’ and a partial reflecting front facet 204 having awidth ‘A’. A distributed feedback grating 206 is disposed in the flaredcurrent injection region 200 so as to extend between the back and frontfacets 202, 204. Distributed feedback grating 206 can have a variablewidth as it extends between the facets 202, 204. Moreover, the grating206 can have a width ‘d’ at the partial reflecting front facet 204 todefine a grating end portion 208 which need not have the same width ‘a’as the high reflecting back facet 202.

While in conventional distributed feedback semiconductor laser diodedevices the width of the grating at the front facet is typicallycoextensive with the width of the front facet and the area of thegrating is coextensive with the pumped area of the diode, in devices inaccordance with the present invention the width ‘d’ of the grating 206can be selected to be the same or preferably narrower than the width ‘A’of the front facet 204. In some examples the width of the grating 206varies along the length of the region 200. Since the grating 206 has asmaller area than the entirety of region 200, the total scattering lossintroduced by imperfections in the grating is reduced, leading toimproved operating efficiency.

In FIG. 12B a top cross-sectional view of a current injection region 210of a flared laser oscillator waveguide device is shown which is alsoconfigured to be wavelength stabilized. The region 210 includes anarrower high reflecting back facet 212 having a width ‘a’ and a partialreflecting front facet 214 having a width ‘A’. A distributed Braggreflector grating 216 is disposed in the region 210 at the highreflecting back facet 212. The grating 216 extends the width ‘a’ of theback facet 212 at the back facet 212, extends a length ‘L_(grt)’ alongthe longitudinal axis of the device, and extends to a width ‘d’ insidethe region 210. As can be seen from FIG. 12B, the width of ‘d’ need notbe equal to ‘a’. In most cases d>a and the width of ‘d’ can stretch allthe way to the lateral dimension of the pumped region at the locationwhere L_(grt) ends. In some examples, the area of the grating 216 iselectrically-pumped with current during operation. The length of thedistributed Bragg reflector grating 216 is selected to provide highreflectivity (>90%).

It is thought that the present invention and many of its attendantadvantages thereof will be understood from the foregoing description andit will be apparent that various changes may be made in the partsthereof without departing from the spirit and scope of the invention orsacrificing all of its material advantages, the forms hereinbeforedescribed being merely exemplary embodiments thereof.

What is claimed is:
 1. A broad area semiconductor diode laser devicecomprising: a multimode high reflector facet; a partial reflector facetspaced from said multimode high reflector facet; and a flared currentinjection region extending and widening between said multimode highreflector facet and said partial reflector facet, wherein the ratio of apartial reflector facet width to a high reflector facet width is n:1,where n>1.
 2. The device of claim 1 wherein said flared currentinjection region propagates light such that a beam output at saidpartial reflector facet has a narrower beam width than said partialreflector facet width and a corresponding narrower slow-axis divergence.3. The device of claim 2 wherein said substantially narrower beam widthand slow-axis divergence in conjunction with an enlarged total pumpedarea provided by the flaring of said flared current injection region areoperable to reduce thermal resistance and electrical series resistanceand result in increased beam brightness and lower beam parameter productof said beam output for a selected device output power.
 4. The device ofclaim 1 wherein said high reflector facet has a width selected in therange of about 10 μm to 200 μm.
 5. The device of claim 1 wherein saidflared current injection region flares with a constant change in widthwith respect to length.
 6. The device of claim 1 wherein said flaredcurrent injection region flares with a variable change in width withrespect to length.
 7. The device of claim 1 wherein said flared currentinjection region flares with a plurality of flare regions.
 8. The deviceof claim 1 wherein said current injection region includes a rectangularend portion positioned at said multimode high reflector facet allowing acleave to be formed in said rectangular end portion providing apredictable width for said multimode high reflector facet.
 9. The deviceof claim 1 wherein said current injection region includes a rectangularend portion positioned at said multimode partial reflector facetallowing a cleave to be formed in said rectangular end portion providinga predictable width for said multimode partial reflector facet.
 10. Thedevice of claim 1 wherein said flared current injection region is gainguided.
 11. The device of claim 1 wherein said flared current injectionregion is index guided.
 12. The device of claim 1 wherein said ratio offacet widths is optimized for brightness over a selected current range.13. The device of claim 1 further comprising a pair of scatteringelements disposed along opposing lateral sides of said flared currentinjection region, said scattering elements operable to scatter higherorder modes of light propagating therein.
 14. The device of claim 1further comprising a distributed feedback grating disposed in saidflared current injected region and extending between said facets, saiddistributed feedback grating providing wavelength stabilization to thedevice.
 15. The device of claim 1 further comprising a distributed Braggreflector grating positioned at said high reflector facet, saiddistributed Bragg reflector providing wavelength stabilization to thedevice.
 16. The device of claim 1 wherein a beam is emittedsubstantially in the fundamental mode across a fast axis thereof. 17.The device of claim 1 wherein said multimode high reflector facet has areflectivity of about 90% or greater.
 18. The device of claim 1 whereinsaid partial reflecting facet has a reflectivity of about 0.05% orgreater.
 19. A multimode flared laser oscillator waveguide comprising: asemiconductor gain volume having a multimode high reflector and anoutput coupler oppositely disposed and spaced apart by a resonatorlength, top and bottom oppositely disposed sides spaced apart by aresonator height, and first and second oppositely disposed flared sidesspaced apart by a variable resonator width providing said high reflectorwith a shorter width than said output coupler.
 20. A flared laseroscillator waveguide comprising: a semiconductor gain volume including:a high reflector surface and an opposing partial reflector surfacespaced apart from each other by a resonator length; top and bottomopposite surfaces spaced apart by a resonator height; and first andsecond opposite side surfaces spaced apart by a resonator width whereinat least a portion of said opposite side surfaces are spaced apart by avariable resonator width forming a flared oscillator region andproviding said high reflector surface with a shorter width than saidpartial reflector surface.